Offshore floating production and storage systems are often used in the recovering and processing of hydrocarbons from geological formations beneath the ocean floor. These systems usually include a production riser system, which provides conduits for transporting produced fluids from the ocean floor to a marine vessel for crude oil processing and storage. The production riser system may also include a method for anchoring the vessel. Production riser systems are particularly useful in water too deep for a production platform or too remote to run a pipeline to onshore processing and storage facilities.
Offshore floating production terminals are also used in the recovering and processing of hydrocarbons from subsea geological formations. Like floating production and storage systems, floating production terminals include a riser system that provides conduits for transporting fluids from the ocean floor to a marine vessel. However, the fluid transported from the ocean floor in a floating production terminal is crude oil which has been processed at another location, such as an offshore fixed platform or an onshore location, and is being pumped to an offshore storage vessel. Both offshore floating production and storage systems and offshore floating terminal systems require a method for anchoring the marine vessel during production or loading and a riser which houses the flowlines carrying the hydrocarbon fluids from the ocean floor to the marine vessel. In some offshore production systems (used hereinafter to collectively refer to both "offshore floating production and storage systems" and "offshore floating terminal systems"), the riser is designed to be part of the anchoring system for the marine vessel. Such systems may be referred to as single point mooring systems. A particular single point mooring system is the single anchor leg mooring ("SALM") system.
A typical offshore production SALM is attached by a universal joint to a base which is fixed to the ocean floor. The base may be a simple anchoring device to which flowlines can be laid from an underwater production manifold, a single wellhead or multiple wellheads. A riser structure housing the required fluid conduits extends up through the water from the universal joint at the base to a buoy which reaches above the water surface. In some SALM installations, especially those in water depths of three hundred feet or more, a second universal joint between the riser pipe and the buoy may be installed. Above the buoy, a mooring swivel and a fluid swivel stack are rotatably mounted on top of the SALM. An example of a fluid swivel stack may be found in U.S. Pat. No. 4,126,336 to Ortloff et al. The fluid conduits carried by the riser structure extend from the base to the fluid swivel stack at the top of the buoy. Flexible components permit the fluid conduits to bend as required at the universal joints as they flex in response to the marine vessel movement. Fluid conduits, connected to each swivel of the fluid swivel stack, transport produced oil and gas from the swivel stack to a marine vessel. The marine vessel is moored to the SALM by a rigid yoke or arm. One end of the rigid arm is attached to the marine vessel. The other end of the arm is fastened, usually by a hinge mechanism, to the mooring swivel of the offshore production system.
To prevent twisting and breaking of the fluid conduits running from the fluid swivel stack to the moored marine vessel, the mooring swivel and the fluid swivel stack are joined so they will rotate together about the longitudinal axis of the SALM buoy. Therefore, as the marine vessel and rigid mooring arm rotate horizontally about the longitudinal axis of the SALM buoy, the end of the mooring arm connected to the mooring swivel causes the mooring swivel and attached fluid swivel stack to rotate about the SALM axis.
To prevent leakage of produced fluids and protect the internal components of each fluid swivel, elastomeric seals are placed in each swivel between the housing and the swivel shaft. The swivel shafts are stationary with respect to the riser and the swivel housings rotate with the vessel as it rotates about the substantially vertical axis of the SALM buoy. Typically, lip-type seals of synthetic rubber, neoprene, fluorocarbon or teflon are used in such applications. However, as the fluid swivels rotate in response to marine vessel movement, these seals wear. Worn seals may leak produced fluids as well as cause bearing failure and impede free rotation of the fluid swivels on the shaft. Replacing fluid swivel seals may result in costly downtime and repair. Reduced rotational movement of the swivel stack would increase fluid swivel seal life by reducing fluid swivel seal wear.